Radio frequency (RF) ablation is widely used for treating cardiac arrhythmias. RF ablation is commonly carried out by inserting a catheter through the patient's vascular system into the heart, and bringing the distal tip of the catheter into contact with the cardiac tissue at the site that is to be ablated. RF electrical current is then conducted through wires in the catheter to one or more electrodes at the tip of the catheter, which apply the RF energy to the myocardium. The RF energy is absorbed in the tissue, heating it to the point (typically about 50° C.) at which it permanently loses its electrical excitability. When this sort of procedure is successful, it creates non-conducting lesions in the cardiac tissue, which disrupt the abnormal electrical pathway causing the arrhythmia.
It is often difficult to determine the proper dosage of RF energy that should be applied in an ablation procedure in order to achieve the desired result. When the dosage is insufficient, the non-conducting lesion will not extend deeply enough through the heart wall to disrupt the abnormal conduction, so that arrhythmia may persist or return after the procedure is completed. On the other hand, excessive RF dosage may cause dangerous damage to the tissue at and around the ablation site. The proper RF dosage is known to vary from case to case depending on various factors, such as catheter geometry, thickness of the heart wall, quality of the electrical contact between the catheter electrode and the heart wall, and blood flow in the vicinity of the ablation site (which carries away heat generated by the RF energy).
In order to improve the precision and consistency of RF ablation procedures, attempts have been made to predict and control the ablation based on measurement of physiological parameters of relevance. One parameter that has been found useful in this context is the electrical impedance between the catheter electrode and the endocardial tissue, as described, for example, by Cao et al., in “Using Electrical Impedance to Predict Catheter-Endocardial Contact During RF Cardiac Ablation,” IEEE Transactions on Biomedical Engineering 49:3 (2002), pages 247-253, which is incorporated herein by reference. The authors note that the electrode-endocardial contact includes two aspects: the depth to which the catheter is inserted into the myocardium and the angle between the catheter and the endocardial surface. These qualities, however, are difficult to ascertain in catheterization systems known in the art, and they may vary during the ablation procedure. The article describes a method for predicting the insertion depth of the catheter using electrical impedance measurements. The use of impedance measurements in controlling RF ablation is also described in U.S. Pat. No. 6,391,024, whose disclosure is incorporated herein by reference.
Various other methods and devices are known in the art for controlling ablation based on physiological measurement. For example, U.S. Patent Application Publication US 2002/0169445 A1, whose disclosure is incorporated herein by reference, describes a RF ablation system in which energy delivery is linked to fluid flow. Fluid flow rate in a biological organ is provided by an ECG device or a flow sensor. A processor assesses whether the flow rate is low or high, and controls the RF generator accordingly. As another example, U.S. Patent Application Publication US 2002/0128639 A1, whose disclosure is also incorporated herein by reference, describes a device and method for forming a lesion, wherein the temperature response of the tissue to be ablated (in this case by focused ultrasound) is measured in advance of the ablation. The temperature response is affected by factors such as tissue thickness, amount of fat and muscle, and blood flow through and around the region in question. The temperature response of the tissue is analyzed to determine the appropriate ablation technique.
The catheter that is used to perform the RF ablation may also be used to observe the results of the ablation. For example, U.S. Pat. No. 5,588,432, whose disclosure is incorporated herein by reference, describes catheters for imaging, sensing electrical potentials and ablating tissue. In one embodiment described in the patent, a sonolucent electrode at the tip of the catheter is used to perform RF ablation. An ultrasonic transducer is positioned to transmit ultrasonic signals through the electrode into the heart tissue, in order to create an ultrasonic image. The imaging capability of the catheter can be used to determine immediately whether a specific change to the tissue has resulted from the ablation. Desiccation of tissue manifests itself as a brightening of the region of the ultrasound image corresponding to the location of the lesion. This brightening corresponds to increased reflection of ultrasonic signals.